November 17, 2013

Evolution Picks Evolvability As A Survival Trait

What is more useful? Having a superpower, or the ability to develop a superpower? Evolution, of course, picks the latter.

Evolution favors the survival of the fittest. If a trait helps an organism adapt to a changing environment, that trait is favored and preserved by evolution.

For example, when insects are affected by a pesticide, some of them develop resistance to it, while others die. Evolution targets these newly-developed traits in organisms, such as pesticide resistance. Only those that have these traits live on and pass them on to their children, so that the species continues to live.

But the very ability of the organism to adapt is a trait that can be passed on to the next generation. And that ability itself can be targeted by evolution.

To understand this better, consider a species of rats living in the desert. Imagine that they are often attacked by predators such as birds which can see them from above. Under such stressful conditions, some rats start developing a sand-colored skin tone to hide better, while others don’t.

Naturally, evolution picks these sand-colored rats and lets them live, while the others end up becoming bird food. In this case, it looks like evolution picked sand-colored skin tone as the factor to preserve.

But how were the rats able to develop that skin tone in the first place? They must have had an innate ability to change their skin color when the going got tough, so to speak. This ability to adapt or “evolvability” itself is a prized factor, more than just having a “safer” skin tone.

In other words, evolution not only wants to preserve the sand-colored skin tone in the rats, but also the ability of the rats to develop that skin tone.

Now scientists have proof to show that evolution also targets this “evolvability.”

"It's not controversial that populations evolve and that some traits are more apt to evolve than others," said Dustin Brisson, biologist at UPenn and senior author of the study. "What we were asking is whether the ability of an organism to evolve is a trait that natural selection can pick."

How did the scientists find out? They took a closer look at bacteria, in particular the one that causes Lyme disease.

Microbes such as bacteria and viruses have long had the ability to adapt quickly - within generations, in fact. While some animals take millions of years to develop a certain trait, these tiny creatures can develop a new trait within weeks or months.

Microbes that infect plants or animals face a lot more challenges to survive because the host’s immune system keeps killing them. They not only need to be strong, but also be able to quickly change their body make-up to become strong in the future, if needed.

In fact, they don’t need to know what the future has in store for them in order to be able to adapt to adverse conditions. Whatever the host’s immune system throws at them, they learn how to adapt to it, without needing to know what it is in advance.

The secret to this resilience, it turns out, lies in the genes. The microbes’ genes give rise to proteins which help infect the host. In the case of the Lyme bacteria, for instance, a particular protein called VlsE on the cell’s outer surface serves this function.

Once infected, however, the host’s immune system identifies the VlsE protein on the bacterial cell surface, realizes that it is a foreign body and produces antibodies that start killing the bacteria.

The bacteria which planned to sneak into the host have had their cover blown, so to speak.

So what do the bacteria do? They quickly tweak their genes to produce a modified version of the VlsE protein that the immune system cannot recognize anymore – within weeks, as some studies have shown.

How are they able to change their genetic structure so quickly?

It turns out that the have a sort of “back-up” set of genes called cassettes. These cassettes remain silent under normal conditions i.e. they do not give rise to proteins or perform any other tasks.

However, when the bacterium realizes that the host has “identified” it, these cassettes quickly exchange parts of their gene structure with the VlsE gene, making the protein look very different from its original structure. (Imagine a back-stage crew helping the actor change his appearance between acts by dressing him up in some of their clothes).

Now the protein looks brand new, and like something that the host doesn’t recognize, so the bacterium continues to live and infect the host.

It was these cassettes that the scientists decided to look at more closely.

When they studied 12 different strains of the Lyme disease bacteria with different cassettes, they found that evolution focuses more on these cassettes rather than the VlsE gene. The more different one organism’s cassettes are from the others, the greater the chances of the species’ survival, they found.

A greater variety of cassettes means a wider range of new VlsE proteins that are created each time to baffle the host’s immune system. In other words, the diversity of the cassettes ensures the “evolvability” of the VlsE protein.

This makes sense because if all the bacteria had the same type of cassettes, and produce the same type of VlsE protein the second time, the host would just kill them all.

They also found that mutations in these cassettes were more often and more common, indicating that a lot of tweaking was going on inside them.

What evolution appears to be doing in this bacteria’s case is changing the cassettes to make them more and more diverse to ensure that the bacteria continue to survive no matter what the host does to thwart them.

This concept might be more difficult to show in other animals or human beings, because of the complexity of their genetic systems, the researchers believe.

"But we can now say that evolvability can be the object of selection in the face of environmental pressure,” Brisson said in the release.